1. Field of the Invention
The present invention relates to a magnetic recording medium for high-density recording which has excellent electromagnetic characteristics.
2. Related Art
Increasing the density and the capacity of a magnetic recording medium is in other words recording a greater amount of information in unit volume. There are three main approaches that have been taken to this subject. The first one is to shorten the recording wavelength, i.e., to improve linear recording density. The second one is to narrow the width of the recording tracks. The third one is to increase the recording area which corresponds to make a recording medium as thin as possible. These three approaches have been practically combined with each other to realize a high recording capacity. The feature of the present invention will be described below together with the technical trend for obtaining an increased density and capacity in magnetic recording media.
At first, the studies were made to magnetic recording media using an analog recording system. The media are required to have higher output over wide recording wavelengths of from long wavelengths to short wavelengths. Proposed technique to attain the requirement was to form two or more magnetic layers. The technique was initially proposed as an audio tape in the 1970s and advanced later as a high-performance video tape. This technique is designed to comprise a lower magnetic layer having a relatively low coercive force and an upper magnetic layer with a relatively high coercive force in order to obtain high output over wide recording wavelengths. In other words, the lower magnetic layer contributes to output at long wavelengths while the upper magnetic layer contributes to output at short wavelengths.
In the next step to develop high density and high capacity recording media, attention has been paid on digital recording system. In comparison with analog recording media, high output at short wavelengths is more essential for digital recording media. A plural layer coating method is commonly applied, wherein layer structure of recorded side has a thin upper magnetic layer on a lower non-magnetic layer. Many studies and inventions have been made to pursue high output of such media. Some examples are shown below.
According to U.S. Pat. No. 5,496,607, if the thickness of the coated magnetic layer is large, problems of self-demagnetization loss in recording and thickness loss in reproduction are serious. Ejiri et al. described that there was an optimum thickness for the output for each frequency, and the optimum thickness was approximately within the thickness range of 1/4 to 1/3 of the recording wavelength at each frequency. (134.sup.th SMPTE TECHNICAL CONFERENCE PRINT No.134-117). Considering the wavelength used for high capacity magnetic media is below 1 .mu.m, the optimum thickness proposed by Ejiri lies below 0.3 .mu.m. In any case, formation of a thin upper magnetic layer is required to get high density digital recording media. After all, the magnetic media having the thin upper magnetic layer on a lower non-magnetic layer is, for the present, authorized as recording media with high density and high capacity.
In order to form plural layers of magnetic layer and non-magnetic layer beautifully, it is preferable to apply a simultaneous coating method or a successive wet coating method, called wet-on-wet coating method (U.S. Pat. No. 5,756,148). According to U.S. Pat. Nos. 5,496,607 and 5,645,917, preferable relations between the upper magnetic layer and the lower non-magnetic layer are described as; (1) there exists an interface between the upper magnetic layer and the lower non-magnetic layer, (2) it is ideal that the interface forms a straight line with a constant thickness of layer, (3) no mixed region is allowed to exist between the lower non-magnetic layer and upper magnetic layer.
As for the dispersions (coating compositions), the dispersion of lower non-magnetic layer and the upper magnetic layer are required (a) to have equal or approximate thixotropy and (b) to prevent dynamic formation of a mixed region at the interface between the upper and lower and layers (U.S. Pat. No. 5,792,543). To accomplish these embodiments, several means are shown in U.S. Pat. Nos. 5,792,543, 5,763,046, and 5,780,141.
Among these means, some modification is proposed, wherein the maximum magnetic flux density Bm of the lower non-magnetic layer is controlled between 30 gauss and 500 gauss. In this modification, the lower non-magnetic layer comprises magnetic powder, but it is regarded as "non-magnetic layer": the magnetic powder in this means is incapable of magnetic recording and gives no adverse infuence on the upper magnetic layer (U.S. Pat. No. 5,792,543). In either case, the lower non-magnetic layers described in these inventions are confined to the layers having no contribution on magnetic recording process.
Apart from the above-described structure comprising an upper magnetic layer and a lower non-magnetic layer, U.S. Pat. No. 5,776,590 discloses a structure comprising an upper magnetic layer and a lower magnetic layer adjacent thereto. The magnetic recording medium disclosed in this patent makes it possible to avoid the reduction of output at short wavelengths and to balance the output in both short and long wavelengths, by means of properly selecting the type and the amount of the magnetic powder to be contained in the lower magnetic layer. This medium contains hexagonal tabular magnetic particles whose axis of easy magnetization is perpendicular to the main plane thereof in the lower magnetic layer at a specific ratio. While the technique according to U.S. Pat. No. 5,776,590 may be able to avoid reduction in output at short wavelengths, the demand for an improved output of a magnetic recording medium of coated type at short wavelengths still remains unfulfilled.
The inventors of the present invention have extensively studied and further developed the technique of U.S. Pat. No. 5,776,590, to further improve the output of a magnetic recording medium of coated type at short wavelengths and found, as a result, that the output characteristics at short wavelengths can be improved when the upper and lower magnetic layers satisfy specific conditions. One of the most important conditions is the orientation of magnetic powder contained in the upper and lower magnetic layers.
Various studies have heretofore been given to orientation of magnetic particles for the purpose of improving output at short wavelengths. For example, Japanese Patent Laid-Open No. 334652/93 discloses a magnetic recording medium in which ferromagnetic powder in the magnetic layer is orientated with its axis of easy magnetization being diagonal at an incline of 10 to 65.degree.. According to this technique, however, the diagonal orientation of the ferromagnetic powder in the upper magnetic layer is accompanied with difficulty in securing surface smoothness of the magnetic layer that is required for achieving high-density recording, and the improvement of output reached is limited. Furthermore, magnetic powders that can be used are limited in kind, and the process for manufacturing the magnetic media is complicated.
In the present invention a state of orientation of magnetic powder with which specific magnetic characteristics are manifested can be accomplished by appropriately combining various conditions of forming the upper and lower magnetic layers, such as the conditions for preparing coating compositions, controlled solid concentrations of coating compositions, and drying conditions. Accordingly, a precisely designed orientating apparatus that has conventionally been used is not always required in the practice of the present invention, and the magnetic recording medium of the present invention can be seen as advantageous for mass production.